Portions to be soldered, such as electrode pads which are formed on the surface of a substrate of an electronic part such as a BGA (ball grid array) package or on the surface of a printed circuit board (PCB), are often made of copper (Cu), since Cu is wettable by molten solder as easily as are noble metals such as gold (Au) and silver (Ag) and it is much less expensive than these noble metals. However, there are cases in which the advantage of Cu that it has good wettability by molten solder is developed excessively so that the formation of an intermetallic compound of Cu with Sn (contained in the solder) takes place during soldering more than is required for the solder to form soldered joints of sufficient bonding strength. The formation of such an intermetallic compound, which is inherently brittle, continues to develop even during aging of the soldered joints.
Therefore, when Cu is used to form portions to be soldered such as electrode pads, it is found that soldered joints formed on such portions are susceptible to brittle fracture due to the formation of a Cu—Sn intermetallic compound in an excessive amount.
In contrast, as is well known to those skilled in the art, if nickel (Ni) is used to form portions to be soldered and soldering is performed under appropriately controlled conditions, it is possible to attain sufficient wettability by molten solder while the amount of an intermetallic compound formed is controlled to a reasonable level required to provide the resulting soldered joints with sufficient bonding strength. However, due to the cost advantage of Cu that it is less expensive than Ni, Cu is preferred as the most versatile nonferrous conductive metal in many applications including electronic parts.
In order to exploit the advantages of Cu and Ni together, substrates for electronic parts having portions to be soldered which are made of Cu and coated with Ni by plating have recently been employed increasingly. Thus, portions to be soldered such as electrode pads have a body of Cu, and only their surface which must be wetted by molten solder is formed by Ni plating so as to prevent the formation of an intermetallic compound in an excessive amount.
Ni plating can be performed either by electroplating or electroless plating. Electroplating forms a metal layer on the surface of a cathode by an electrolytic reaction. When a substrate such as a substrate of a BGA package or of a PCB is subjected to electroplating to form a Ni layer on portions to be soldered (i.e., electrode pads) of the substrate, there are many problems such as the necessity of electrical connection to each of the tiny electrode pads so as to allow them to act as cathodes, a limitation on the current density for electrolysis, and corrosion of metallic portions in a plating bath. A soluble anode, which is burdensome in replacement when it has been consumed and which is wasteful since it cannot be consumed completely, is generally used in electroplating with Ni. Furthermore, the rate of Ni deposition by electroplating is not uniform, and it is decreased particularly in those areas which are far from the anode or which form recesses.
In contrast, electroless Ni plating is advantageous in that a Ni layer having a uniform thickness can be formed, regardless of the type or shape of a material to be plated, by merely immersing the material in a plating solution without passing an electric current therethrough. Therefore, electroless plating is predominantly employed in Ni plating of substrates.
A typical plating solution which is used in electroless Ni plating is a solution called a Ni—P plating solution, which contains nickel sulfate as a Ni ion source and sodium hypophosphite as a reducing agent. An Ni—P plating solution also contains sodium hydroxide to maintain the pH of the solution at a predetermined value. A nickel coating formed by electroless plating with a Ni—P solution usually contains phosphorus (P) in an amount of 2–15 mass %.
After a substrate of a BGA package (hereunder referred to as a BGA substrate) is subjected to electroless Ni plating to form a Ni layer on the electrode pads of the substrate, flash plating with Au is applied in most cases in order to protect the Ni-plated layer from a chemical reaction such as oxidation and improve the wettability by or affinity for molten solder. Thereafter, solder bumps are formed on the Ni-plated electrode pads of the BGA substrate. When the BGA package is mounted on a PCB (printed circuit board), the solder bumps are allowed to melt to solder the BGA substrate to the PCB. The PCB also has electrode pads, which are typically made of Cu. The Cu electrode pads of the PCB may also be plated with Ni, and the resulting Ni layer may be subjected to flash plating with gold or another noble metal.
The formation of solder bumps on the electrode pads of a BGA substrate is performed by a process comprising applying a sticky flux to the electrode pads, then placing a solder ball on each electrode pad, and heating the BGA substrate having the solder ball on each electrode pad in a reflow furnace at a temperature sufficient to melt the solder ball and convert it into a solder bump bonded to the electrode pad.
The solder balls used to form solder bumps on a BGA substrate are generally made of a solder having a composition which is close to an Sn—Pb eutectic alloy, i.e., a composition consisting essentially of 60–64 mass % of Sn and a remainder of Pb. Such a eutectic or nearly eutectic Sn—Pb solder has a relatively low melting point of at most 190° C., so it is possible to minimize thermal effect on the BGA substrate and the electronic device mounted on the substrate during heating to form the solder bumps and/or subsequent heating for soldering the substrate to a PCB. Another advantage of the Sn—Pb alloy is that it generally exhibits good wetting power.
When solder bumps made of the eutectic or nearly eutectic Sn—Pb solder are formed by the above-described process on Ni-plated electrode pads which have been treated by flash plating with Au, it is expected that in the final heating step in the process, the solder balls which have melted can wet the underlying electrode pads smoothly and form solder bumps firmly secured to the electrode pads due to the Ni plating and flash Au plating and in view of the nature of the solder.
In fact, however, with electronic products having assembled therein a PCB on which a BGA package having solder bumps formed in this manner on its substrate is mounted, the BGA substrate is sometimes detached from the PCB by accident. For example, electronic products such as cellular phones and notebook computers may receive mechanical shocks when a cellular phone put in a pocket of ones clothing slips out of the pocket or when one drops a bag containing a notebook computer from his grasp. In such situations, detachment of soldered joints formed from the above-described solder bumps to connect the BGA substrate to the PCB may occur, leading to failure of electrical connection between these two parts and malfunction of the electronic product.